I'd probably wax poetic.

"I am that which steals life, the violent delight that begets violent ends."

"I am the dagger, the sharpened stone, that unleashes an endless tide of blood."

"I am the cold wind that encounters the flickering candle... and snuffs it out."

I'd probably do something along those lines, but also with an allusive eye toward famous murderers throughout literature, mythology and history. Note that these don't have to be references the PCs are familiar with - a god of Murder might know about Jack the Ripper or Cain or Procrustes, even if the PCs don't have a clue who he's talking about, it'll just sound menacing.